Laser driven color printers and copiers employ transparent toners which enable light to reflect off the page and to be directed back towards the eye. In general, such devices employs Cyan (C), Magenta (M) and Yellow (Y) toners as the principal component colors, from which other colors are created. Light passing through CMY toners has part of its color filtered out or absorbed by the toner such that the reflected light takes on the color of the toners that it passes through. In laser printers (and some copiers), a black (K) toner is used which is opaque to light. If a K toner is overprinted onto CMY to achieve darker colors, such as found in shadows, much of the colorfulness of the shadows is lost. Using K toner exclusively to achieve dark colors also results in much less colorful dark colors because less surrounding colorant is used.
Thus, to increase the range of colors available from a printer/copier, it is necessary to find a correct balance of K and CMY toners to produce dark colors, while allowing the dark colors to remain as colorful as possible.
While the prior art has employed combinations of the three primary colors cyan, magenta and yellow (CMY) to produce darker colors ranging to black, the need to deposit 100% density toner layers for each color causes an excessive amount of toner to be deposited on the media sheet (e.g. up to 300%). Such a high level of toner deposition does not fuse well and, in general, creates unsatisfactory images. Accordingly, prior art printing procedures have utilized combinations of K and CMY toners to achieve dark or shadowed color images.
When a printer receives image data from a host processor, the data is received in the form of either RGB or CMY values. In either case, the received values are converted to CMYK values in order to achieve desired levels of color representation on the final printed document. Such conversion, depending upon the color value parameters that are set by the printer manufacturer, can result in variable color representations when printers of different manufacturers are coupled to a host processor.
Accordingly, it is an object of this invention to provide an improved method for converting primary printer color values to combinations of the primary printer color values with black to achieve improved printer output color representations.
It is another object of this invention to provide an improved method for converting CMY to CMYK, while retaining a maximum gamut of available colors.
It is a further object of this invention to provide an improved method for converting CMY to CMYK, while maintaining toner quantities within determined limits.